1. Field of the Invention
The present invention relates to supplying a load, such as an arrangement of one or more light-emitting diodes (LEDs), with a current whose intensity varies as a function of the temperature of the load.
2. Background Art
A current source supplies current to a light-emitting diode (LED) arrangement having one or more LEDs arranged in a chain, an array, etc., for operation of the LED arrangement. Usually, the supplied current is constant. A problem with supplying constant current to a LED is that the current flow through the LED changes as a function of the temperature of the LED. Hence, the brightness of the LED changes as the temperature of the LED changes while the current supplied to the LED is constant. Consequently, current supplied to a LED must vary as a function of the temperature of the LED in order to keep the light emission yield constant. Typically, constant current from a current source is pulse-width modulated to produce the varying current (i.e., pulse-width modulated current) to be supplied to the LED.
The requirements imposed on such a current source deployed in an automotive application for supplying current to a LED arrangement are high. Automotive applications present strong fluctuations in the operating voltage (e.g., between 8 and 18 volts) of the current source. Automotive applications also present an environment having a broad temperature range (e.g., between −40° C. and +120° C.). Additionally, Additionally, high efficiency is required. Thus, highly elaborate and expensive switching regulators are used in conjunction with the current source to produce a current whose intensity varies as a function of the temperature of the LED arrangement.
Economical analog circuits have been used when the difference between the temperature-dependent LED voltage and the operating voltage is small and compromises in the color temperature are acceptable. This is easier with colored LEDs than with white LEDs.
As the temperature of a LED drops, its forward voltage and luminous efficiency rise. The temperature-dependent rise in the forward voltage can be compensated by reducing the current as the temperature decreases, so that this voltage hardly changes, without having to accept a reduction in the luminous efficiency. However, pure voltage stabilization is not sensible to use due to the dispersion of forward voltages among different specimens. Furthermore, predefined characteristic curves are required from one application to another, which range from positive linear, neutral, negative linear, all the way to nonlinear.
DE 197 32 828 C2 describes a current source having a pulse-width modulation switch coupled to an LED arrangement through inductance.
EP 1 278 402 B1 describes a temperature-dependent current source for supplying current to LEDs. A controller controls the current supplied to the LEDs with a set-point input to which a current setting is fed. A set-point generator outputs a current set-point at a set-point output. A thermometer supplies a measured temperature variable that is linearly dependent on the ambient temperature. A subtractor having first and second inputs and an output subtracts an electrical variable at its second input from an electrical variable at its first input and supplies the result at its output. The current set-point is fed to the first input and the output with the set-point input are connected with the controller. A shut-off device with a shut-off input changes the current setting such that the current through the LEDs becomes negligible when a shut-off signal is applied at the shut-off input. A disadvantage is that the controlled variables have to be continuously monitored and the output values have to be adapted as a function of the controlled variables. As such, this solution is relatively elaborate, many components are used, and is cost-intensive.
DE 199 12 463 A1 (corresponding to U.S. Pat. No. 6,807,202) describes a method for stabilizing the optical output power of LEDs. The method compensates for the influence of temperature on the optical output power of LEDs without sensing the temperature or light output of the LEDs. The method is based on knowledge that the current flowing through a LED and the conducting-state voltage drop across the LED are, at constant light output, independent of the temperature, and that this functional correlation can be determined. If this correlation is known, then the current and conducting-state voltage drop must achieve this correlation in operation in order to eliminate the temperature effect on light output. Thus, the combination of LED current and conducting-state voltage is used as a unique measure of the emitted light output and on the basis of this data the conducting-state voltage as a function of the LED current is used. A disadvantage of this method is that the temperature of each LED is held constant using a Peltier element which makes it expensive to implement the arrangement.
DE 198 10 827 A1 describes a temperature-dependent current source for supplying current to a LED. A logic circuit controls the current source as a function of the LED temperature which is continuously monitored. Switching regulators control the current. The LED current is linearly adapted so that the luminous efficiency decreases exponentially. The use of switching regulators is a cost-intensive solution and therefore is unsuitable for mass application.
DE 103 59 196 A1 describes a light emission arrangement for a vehicle illumination system. A LED and a dropping resistor in series are supplied with a supply voltage and a total current. Current flow simulation means are connected in parallel to the LED. A control unit monitors the supply voltage and controls the current flow simulation means such that a minimum total current flows starting from when the voltage falls below a threshold until a minimum supply voltage is reached. The arrangement is not useful from an energy perspective as current reduction is achieved by parallel loads.
DE 10 2006 033 233 A1 describes supplying current to a LED. The temperature of the LED is determined by tapping and evaluating a forward voltage of the LED in an operating state. A disadvantage is that the temperature of the LED must be continuously monitored as the supplied current is varied on the basis of the temperature.
DE 10 2007 003 343 A1 (corresponding to U.S. Publication No. 2008/0068298) describes a process for operating a LED for a constant light output. The process includes: a) set up a direct voltage from a direct voltage source; b) provide a LED circuit to produce a LED current to make the LED emit light, the LED current being controlled; and c) provide a constant voltage and constant current controller to clamp the LED voltage differential and the current, even under adverse conditions such as voltage spikes. A disadvantage is that temperature monitoring is necessary which makes the circuit engineering expensive to implement.